Square pitched, series-parallel interconnection of solid oxide electrolyte fuel cells is well known, and taught in U.S. Pat. Nos. 4,490,444 and 4,833,045 (Isenberg and Pollack et al., respectively). The fuel cells used contain an air electrode coating on a cylindrical, porous support tube, where the air electrode is covered over about 300 degrees by a solid electrolyte film. Thus there is a 60 degree wide axial strip down the length of the cell. This remaining 60 degrees of air electrode surface is covered by an interconnection strip, as described in U.S. Pat. No. 5,108,850 (Carlson et al.). As a top layer, fuel electrode covers the solid electrolyte over 280 degrees of the electrolyte surface.
These cylindrical cells are placed in a square pitch, series-parallel connected array, wherein the air electrode of one cell is connected to the fuel electrode of the adjacent series-connected cell by virtue of plated interconnection strip and a strip of sintered nickel felt. Other nickel felts provide parallel connections between the fuel electrodes of adjacent cells. The series path is essential for the generation of a practical D.C. stack voltage. The parallel connections provide paths by which the current can circumnegotiate an open circuit cell. Fuel flows axially in the passages formed between the quadrants of groups of four cells. The voltage across two rows of solid oxide fuel cells is approximately 2 volts. It is highly desirable to find a configuration that would increase the row voltage.
During the processing of fuel cells of the above described configuration there is a strong tendency for the cell to bow in such a manner that the interconnection strip lies along the outside of the curvature. It is postulated that this bowing is caused by densification of the air electrode material which is unmasked and therefore directly exposed to the metal chloride vapors during the pore closure phase of the interconnection growth. Very precisely regulated processing conditions minimize the bowing phenomenon and about cells 35 cm long, with thick support tubes, can easily be made with a quite acceptable bow. Much greater difficulty has been experienced with cells having an active length of 50 cm. The existence of significant bow makes it difficult to fabricate individual cells into bundles of cells.
In addition to the standard cell design described previously, unexamined Japanese Patent Publication No. 63-768 (Kaneko et al.) taught interconnections arranged in symmetrical positions, 180 degrees from each other, on the circumference of the oxygen electrode. This design alleviated cracking of the support, oxygen electrode, and electrolyte. Interconnecting this type cell to make an array or bundle would, however, appear difficult. Another design uses a completely circumferential interconnection, as taught by U.S. Pat. No. 4,791,035 (Reichner), which can result in either a square pitch or triangular pitch array. This design, however, substantially decreases gaseous fuel flow access between the cells to the outer fuel electrodes.
What is needed is an open array of fuel cells having increased row voltage and length, which cells would be uniform and substantially straight. It is one of the objects of this invention to provide such an array.